This invention described herein is a method and the system to practice that method used to continuously monitor the alpha activity of a radioactive gas. 2. Description of the Prior Art
When radon gas diffusing from the walls of a uranium mine decays it goes through a series of solid radioactive isotopes known as radon daughters. Particles of dust or moisture may carry these isotopes to the lungs of miners. To reduce this radioactive inhalation and its possible carcinomatous effect it has been suggested that the mine walls be coated with various types of radon gas sealants. Since the radon concentration can continually vary due to factors like the surface barometric pressure, the mine air pressure, the mine temperatures, the mine humidity, and the blasting in the mines, it was decided that continuously monitoring the radon concentration levels with a system would most accurately determine the effectiveness of the sealants under investigation.
Presently used methods and apparatus for monitoring radon by alpha particle concentration levels can be divided into three broad categories: the Lucas flask way, the radon survey instrument way, and the two filter method. Each has its own advantages and disadvantages the most notable disadvantage for our purposes being the inability to monitor continuously. Either a person has to take data all the time or a batch sample has to be analyzed. The Lucas flask method can be considered a batch sampling technique whose operational parameters can be found in the September, 1959 article of Review of Scientific Instruments (volume 28, No. 9 ) entitled "Improved Low-Level Alpha Scintillation Counter for Radon" by H. F. Lucas. In the April 1974 article of Health Physics (Vol. 28, pp. 472-474) entitled "Rapid Measurement of Radon, Decay Products, Unattached Fractions, and Working Level Values of Mine Atmospheres" by A. L. Hill, the details of the operation of the second of these three methods is explained. The two filter method is also a batch sampling method which takes into consideration the flow rate of a sample, the alpha activity on a back filter, and surface area of its cylinder. Another Health Physics publication, volume 18, 1970, on pages 113-122 entitled the "Study of the Two-Filter Method for Radon 222" explains this method. What we have done over these methods and the known prior art is to develop an improved system and method which continuously monitors radon concentration levels. Three relevant references of interest are known.
Two of these are the U.S. Pat. No. 2,878,390 to F. K. Campbell and U.S. Pat. No. 2,898,800 to G. Bergson. The Campbell invention is used to detect the alpha, beta and gamma radiation from radon gas absorbed in a charcoal filter. However, in such an arrangement it would be impossible to separate out the concentration of alpha particles or to continuously monitor their concentration. In the Bergson invention gas is continuously monitored by spectrographic absorption to determine its gaseous components. It is not concerned with detecting the concentration of alpha particles or any radioactive material nor is its detecting device capable of operatively being substituted for our scintillation detector cell.
The third relevant reference of interest is the device described in IEEE Transactions on Nuclear Science Vol. NS-22, No. 1, February 1975, "Design of a Continuous Digital-Output Environmental Radon Monitor" by Wrenn, Spitz and Cohen. This device is designed to measure "environmental levels of radon 222." Typical purposes include indoor air sampling (residential monitoring). While this monitor most nearly approximates our unit, significant differences exist. The monitor of Wrenn, Spitz and Cohen uses passive diffusion through an open-celled foam for sample exchange and electrostatic accumulation of signal enhancement to accommodate low radon levels. Our monitor uses forced-flow sample exchange in view of the relatively high radon concentrations it was designed to measure in uranium mines.
The primary object of this invention is an improved method and system for monitoring alpha activity.